The present invention relates generally to hydraulic circuitry and, more particularly, to a hydraulic circuit for isolating a lifting cylinder from a secondary hydraulic rollstop cylinder in a wheelchair lift mechanism.
Hydraulic lifting devices for transporting wheelchairs into and/or out of automotive vehicles are well known in the art. Typically, these devices include a movable platform and either a hydraulic or a mechanical mechanism for folding and unfolding the platform to and from a stored position and for moving the platform between a street level and a vehicle entry level. Known wheelchair lifting platforms also typically include a rollstop gate, which is a moveable gate that, when engaged, prevents the wheelchair from accidentally rolling off of the platform while it is raised or in motion. The rollstop gate may also be either mechanically or hydraulically actuated.
Mechanically actuated rollstop gates depend upon the movement of the platform to deploy (close) and/or fold (open) the rollstop gate. Such mechanically actuated rollstop gates have certain disadvantages, including a tendency of the gate to stick in the open position while the lift is off of the ground, as well as the general requirement that the platform move up from the ground several inches in order to actuate the closing of the gate.
In contrast, hydraulically actuated rollstop gates may be opened and closed independently of the motion and/or position of the platform because the rollstop gate can be fluidically coupled to a set of hydraulic cylinders. The positive pressure in the cylinders can also be used to lock the gate in the closed position, enhancing wheelchair passenger safety.
One disadvantage of a first type of known hydraulic wheelchair lifting platforms using a single hydraulic circuit to control both the platform lifting operations and the rollstop gate opening and closing operations becomes apparent when the extended end of the platform engages an obstacle, such as a curb, during lowering. This positioning of the platform is known as xe2x80x9cbridgingxe2x80x9d. When the lowering process is completed, the platform becomes tilted towards the vehicle, requiring the wheelchair to travel uphill to disembark. If only one corner of the extended end of the ramp becomes hung up on an obstacle, the ramp may twist, making disembarkment difficult. These problems may be addressed by locking the hydraulic lifting cylinders to keep the platform level upon prematurely engaging a curb or other obstacle before the lowering cycle is completed; however, the rollstop gate then cannot be opened because the gate is hydraulically coupled to the locked hydraulic lifting cylinders. In such a situation, it is impossible to move hydraulic fluid out of the secondary hydraulic rollstop cylinder to open the rollstop gate when the lifting cylinder is hydraulically locked into position, because the known single-circuit wheelchair platform lifting systems are unable to hydraulically isolate the lifting cylinders from the secondary hydraulic rollstop cylinder such that the secondary hydraulic rollstop cylinder may be hydraulically actuated independently of the lifting cylinders. The wheelchair passenger is thus prevented from exiting the bridged platform that has not completed its prescribed descent cycle.
A second type of known hydraulic wheelchair lifts use a dual set of hydraulic circuits, one controlling the lifting function and one controlling the rollstop function. The disadvantage inherent in such systems is that the requirement of independent hydraulic circuits for separately controlling the folding/unfolding operation, the raising/lowering operation, and the rollstop opening/closing operation increases the complexity, bulk, and cost of the lifting platform. None of the known systems isolate the above-mentioned functions absent the use of multiple, independent hydraulic circuits.
It is advantageous to combine these functions into a single hydraulic system to minimize the bulk, weight, complexity and cost of the wheelchair-lifting platform. It is more advantageous to be able to operate the combined functions independently of each other. For example, the elevation of the ground may not always be a constant, requiring the wheelchair passenger to stop the lift at different elevations relative to the vehicle""s interior. The wheelchair passenger must still be able to open the rollstop gate to disembark the platform, independent of whether the platform is fully lowered or not. There is therefore a need for an improved wheelchair lift hydraulic circuit capable of independently isolating and controlling folding and unfolding operations, raising and lowering operations, and rollstop engaging and disengaging operations. The present invention addresses this need.
One embodiment of the present invention relates to a hydraulic circuit capable of independently controlling the actuation of a plurality of hydraulic cylinders. One embodiment of the present invention relates to a hydraulic circuit for independently controlling the stowage functions, vertical movement, and rollstop operations of a wheelchair-lifting platform. Another more preferred embodiment of the present invention relates to a wheelchair lifting platform having a lifting cylinder and a rollstop cylinder and incorporating a hydraulic circuit capable of independently controlling the respective cylinders.